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Simple synthesis of graphitic porous carbon by hydrothermal method for use as a catalyst support in methanol electro-oxidation
Catalysis Communications 10 (2008) 267–271
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Simple synthesis of graphitic porous carbon by hydrothermal method for use as a catalyst support in methanol electro-oxidation Ji Bong Joo a, You Jung Kim a, Wooyoung Kim a, Pil Kim b, Jongheop Yi a,* a
School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul National University, Shinlim-dong, Kwanak-gu, Seoul 151-742, South Korea School of Environmental and Chemical Engineering, Specialized Graduate School of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Deokjin-dong, Deokjin-gu, Jeonju 561-756, South Korea b
a r t i c l e
i n f o
Article history: Received 8 March 2008 Received in revised form 15 August 2008 Accepted 20 August 2008 Available online 12 September 2008 Keywords: Carbon Electrocatalyst Hydrothermal method Graphitic Methanol
a b s t r a c t Graphitic porous carbon (GP-carbon) was prepared using non-toxic and economical carbon precursor (sucrose) by a simple hydrothermal method under a relatively low pyrolysis temperature. Uniform silica particles (100 nm) were used as pore-forming templates and etched by HF solution after carbonization. The GP-carbon exhibited highly graphitic properties with surface area of 425 m2/g and pore volume 0.42 cm3/g. The GP-carbon showed superior performance as a catalyst support in methanol electro-oxidation. The enhanced activity would be closely related to advantageous properties resulting from highlydeveloped graphitic structure of the GP-carbon. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Graphitic carbons have attracted much attention by many scientists and engineers in practical and fundamental researches for use in hydrogen storage, catalysis and electrochemistry, due to their high electronic conductivity and mechanical strength. Practically, graphitic carbon materials, such as graphite powder, carbon nanotube (CNT) and graphitic carbon nanofiber (GNF), have high conductivities and unique electronic properties which affect electron transfer and metal orientation. They showed the enhanced electrochemical performance in many reactions [1,2]. Although there are superior electronic properties and electrochemical performance, the use of the graphitic carbons have been limited due to the disadvantageous characteristics, such as low surface area and uncontrollable morphologies [3]. In addition, synthetic methods are complex and preparation conditions are uneconomical and severe. Thus, many researchers have made effort to fabricate graphitic carbon with various morphologies using economical methods. Generally, graphitic carbons have been prepared by pyrolysis using graphitizable carbon precursors such as polymer [3], aromatic hydrocarbon [4] or mesophase pitches [5,6] at an extremely severe condition. Yu et al. reported that ordered macroporous carbon with a graphitic framework was synthesized from a mesophase pitch as carbon precursor through pyrolysis at 2500 °C [6]. Although the ordered macroporous graphitic carbon was prepared, the pyrolysis * Corresponding author. Tel.: +82 2 880 7438; fax: +82 2 885 6670. E-mail address: [email protected] (J. Yi). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.08.031
temperature was still high. Several research groups have also reported preparation methods for graphitic carbons under mild conditions. Hyeon and co-workers prepared graphitic carbon nanocoils by carbonizing resorcinol–formaldehyde gel doped with metal salt, at a relatively low temperature. The prepared graphitic nanocoil showed higher performance than commercial carbon black for use as catalyst support in methanol electro-oxidation [7]. Hydrothermal synthesis is a simple and easy process for the preparation of powder materials using aqueous solution as a starting material. This method is also suitable for the control of crystal growth and mass production at a low temperature. Recently, nanostructured carbon materials, such as carbon sphere and porous carbon, have been synthesized by the hydrothermal synthesis route [8–10]. However, few investigations have been reported for the preparation of graphitic carbon materials which have a variety of morphologies. In this work, an economical and easy method for the preparation of GP-carbon under mild synthesis conditions is reported. We prepared highly graphitic GP-carbon using a simple hydrothermal method with templating route. The resulting material was characterized by SEM, HR-TEM, XRD and Raman spectroscopy, and examined for use as a catalyst support in methanol electro-oxidation.
2. Experimental The GP-carbon was prepared by a simple hydrothermal method using uniform-sized silica particles as templates under mild
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pyrolysis conditions. Uniform-sized silica particles were synthesized as described by Stöber et al. [11]. Fig. 1 illustrates the preparation scheme for GP-carbon synthesized in this study. Silica particles (100 nm diameter) were well dispersed in de-ionized water by ultrasonication for 3 h. Sucrose (Aldrich) and iron salt (Fe(NO3)3 9H2O, Sigma) were dissolved into the above aqueous solution under vigorous stirring. The aqueous solution was charged in a teflon-coated stainless steel autoclave. The autoclave was placed in a convection oven at 190 °C for 10 h with vigorous stirring. After cooling down the autoclave at room temperature, the black precipitates were filtered and dried to obtain polymer–silica composites. These polymer–silica composites were carbonized at 900 °C for 5 h and then carbon–silica composites, which contained the graphitic structure, were formed. Final GP-carbon was obtained by dissolution of the silica template and removal of Fe species with diluted HF and HNO3 solutions, respectively. For comparison purposes, sucrose was polymerized under ambient conditions without hydrothermal treatment to obtain amorphous porous carbon (APcarbon). The supported Pt catalyst (Pt/GP-carbon) was prepared by formaldehyde reduction method reported by Zhou et al. [12]. The Pt precursor (H2PtCl6, Acrose) was dissolved in 500 ml of de-ionized water and the prepared carbon support was well dispersed by ultrasonication. The concentrated formaldehyde solution as a reducing agent was added dropwise to the above solution and the batch was heated up to 70 °C under vigorous stirring for 5 h. The black precipitates were filtered, washed with copious amount of de-ionzed water and dried at 80 °C in vacuum conditions. The Pt loading was adjusted to 60 wt.% in the final catalyst. X-ray diffraction (XRD) patterns were recorded using a M18XHF-SRA diffractometer (MAC science). The macroscopic morphology and lattices of carbon support were investigated by field emission scanning electron microscopy (FE-SEM, JEOL, JSM6700F) and high resolution transmission electron microscopy (HR-TEM, JEOL, JEM-3010). Raman spectroscopy (Raman, RM 1000 Renishaw) was carried out to investigate the degree of crystalline structure of carbon support. The electrocatalytic activity in methanol electro-oxidation was investigated in a conventional half cell by cyclic voltammetry using EG&G 263A potentiostat. Pt gause and saturated calomel electrode (SCE) were used as a counter and a reference electrode, respectively. The working electrode was prepared by coating the catalyst ink on a disk-type glassy carbon electrode. Cyclic voltammograms
were recorded in 0.5 M H2SO4 solution containing 1 M CH3OH at room temperature. 3. Results and discussion Fig. 2 shows SEM and HR-TEM images of the prepared GP-carbon. As shown in Fig. 2a, partially broken pores were randomly distributed in the carbon framework, and the carbon had an openpore structure. The spherical pores with an average pore diameter of 100 nm were easily observed in the high magnitude SEM image (Fig. 2b). As described in Fig. 1, pores were formed by silica-templating route. During the hydrothermal process, the sucrose was polymerized on the surface of randomly distributed silica particles in the synthetic solution and silica templates were located in the polymer framework after polymerization of sucrose. After carbonization and silica etching, silica particles were removed by HF solution and regular spherical pores were generated. It should be noted that the pore of support material has an important role in many catalytic reactions. In electrocatalytic reaction, large pore of carbon support has many advantages. It is well known that electrocatalysis in a electrochemical system, such as fuel cells and battery, is taken place in the interfaces of the reactant, the catalyst surface and the electrolyte, i.e., a triple-phase boundary [13]. The triple-phase boundary would be easily formed in the relatively large pore, compared to micropore of support materials because the conventional polymer electrolyte has large molecular size. Thus, the carbon support with regularly large pore is more desirable in view of the mass transfer and would show considerably high performance in practical fuel cells. The (0 0 2) lattice fringes of the graphitic structure are clearly observed in HR-TEM images indicating that GP-carbon has highly graphitic characteristics (Fig. 2c). The d002 was measured and 0 found to be ca. 3.4 Å A. In addition, the GP-carbon exhibited a relatively high surface area (425 m2/g) and a large pore volume (0.42 cm3/g). X-ray diffraction patterns of the prepared carbons are shown in Fig. 3. In the XRD patterns of Vulcan XC-72 (Cabot) and AP-carbon, a broad peak at ca. 2h = 25° was observed, while the GP-carbon showed a sharp peak at the same angle which was caused by the (0 0 2) diffraction plane of the hexagonal structure of graphitic carbon [9,10]. The graphitic crystalline size was measured using Scherer formula and0 d002 spacing for G-carbon was calculated to be 9.8 nm and 3.35 Å A, respectively. The d002 spacing value of G-car-
Sucrose
Silica Fe species
Hydrothermal treatment and carbonization
GP- carbon
Silica etching
Fig. 1. Scheme of preparation procedure for GP-carbon.
Graphitization
J.B. Joo et al. / Catalysis Communications 10 (2008) 267–271
269
Fig. 2. (a), (b) SEM image and (c) HR-TEM image of the prepared GP-carbon.
Fig. 3. X-ray diffraction patterns of GP-carbon, AP-carbon and Vulcan XC-72.
bon is similar to that investigated from HR-TEM. These results imply that GP-carbon is a graphitic carbon with high crystallinity, while Vulcan XC-72 and AP-carbon are amorphous carbons with few graphitic properties [14]. Generally, graphitic structure was generated by pyrolysis of aromatic polymer under high temperature. In this work, porous carbon with graphitic framework was prepared under mild synthetic conditions. From experimental results, it could be noted that the polymerization of sucrose and subsequent aromatization were occurred through the catalytic action of Fe species during hydrothermal treatment. The graphitic structure of GP-carbon was effectively formed by a subsequent pyrolysis step. Hydrothermal treatment has also been reported to induce aromatization of polymers [15], and polymers have been easily converted to graphitic carbon in the presence of metal or metal oxide [16]. Graphitic characteristics of GP-carbon were further confirmed by Raman spectroscopy (Fig. 4). For comparison purposes, the Raman analysis of multi-wall carbon nanotubes (MWCNTs), which have highly graphitic properties, was carried out under same conditions. All carbon materials exhibit two distinct bands at around 1350 cm1 (D-band) and 1580 cm1 (G-band), while a peak at ca.
2720 cm1 (G0 -band or 2D-band) was observed in GP-carbon and MWCNTs. The D-band is closely related to the disordered scattering, which results from the imperfection or loss of hexagonal symmetry of the graphite structure. However, the G-band is observed in both amorphous carbon and graphite [17,18]. It should be noted that the G0 -band is only observed in highly-developed graphitic carbon [19]. Generally, the ratio of the D-band area to the G-band area (AD/AG) is expressed as an index of the degree of graphitization [18,20]. The AD/AG values of GP-carbon, MWCNTs, Vulcan XC-72 and AP-carbon were 1.1, 1.03, 1.65 and 2.1, respectively. The AD/AG value of GP-carbon was very close to that of MWCNTs. Importantly, the G0 -band peak was clearly observed in GP-carbon, and the intensity of the G0 -band was high as seen in Fig. 4. This indicates that GP-carbon had highly graphitic characteristics and retained its graphitic properties, which are comparable to those of MWCNTs. The feasibility of the prepared GP-carbon for the use as a catalyst support in methanol electro-oxidation was investigated. Fig. 5 shows a representative cyclic voltammogram of the Pt/GP-Carbon (inset figure) and voltammograms of the prepared Pt catalysts during the anodic scan. As shown in inset image, the current for
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Fig. 4. Raman spectra of GP-carbon, AP-carbon, MWCNT and Vulcan XC-72.
Fig. 5. Cyclic voltammograms of the prepared Pt catalysts (60 wt% loading) in 0.5 M H2SO4 containing 1 M CH3OH (scan rate = 30 mV/s).
methanol electro-oxidation slowly increased between 0 V and 0.3 V. The current increased dramatically after 0.3 V and maximum peak current was observed at ca. 0.7 V. The current then decreased because the number of active sites for methanol electro-oxidation had declined due to the formation of Pt oxide. After 0.8 V, the current re-increased, indicating that methanol electro-oxidation was occurring on the Pt-oxide surface. In the cathodic scan, the active sites were recovered due to the reduction of Pt oxide, which produced a methanol electro-oxidation current peak. The catalytic activities of electrocatalysts were generally evaluated by comparing the onset potential and current density on the anodic sweep. In this work, catalytic activities in methanol electro-oxidation were
compared using the current density during the anodic sweep. The Pt/AP-carbon had a slightly lower current density than the Pt-commercial catalyst (ETEK). However, the Pt/GP-carbon had the highest anodic current density among Pt catalysts. These results indicate that Pt/GP-carbon have the highest catalytic activity in methanol electro-oxidation. Since same amount of carbon-supported Pt catalyst, which has identical amount of Pt on the catalyst, was used on the working electrode, the variation in catalytic activity can be explained by the attribution to the effect of support material. Carbon materials with a graphitic structure have been frequently used in low temperature fuel cells and showed the
J.B. Joo et al. / Catalysis Communications 10 (2008) 267–271
unexpected enhancement in catalytic performance. The exact reason of the enhancement has been still controversial. However, it is believed that the unique electronic properties and the crystallographic orientation of graphite structure had positive effects on electrocatalysis. In this work, graphitic porous carbon was easily prepared using sucrose under mild synthetic conditions and showed the superior performance as a catalyst support in methanol electro-oxidation. From experimental results, it can be concluded that the highly graphitic nature of GP-carbon had positive effects on catalytic function of supported Pt catalyst, resulting in the high catalytic activity of Pt/GP-carbon in methanol electrooxidation.
4. Conclusions GP-carbon was prepared by a simple hydrothermal method at a relatively low carbonization temperature. The prepared GP-carbon was used as a catalyst support for a Pt catalyst in methanol electrooxidation. The GP-carbon exhibited highly graphitic characteristics and Pt/GP-carbon showed the superior performance in methanol electro-oxidation. The high activity is closely related to the unique properties of graphitic carbon, which affect on positive effect on catalytic function. The preparation method suggested in this work is a simple and efficient process for the preparation of graphitic carbon in mild conditions using low-cost materials, such as sucrose, which is non-toxic and easy to handle. Therefore, the method described in this work would be an easy and environmentalfriendly technique for the production of graphitic porous carbon.
271
Acknowledgement We thank Prof. J. Choo at Han-Yang University for the Raman measurements. References [1] C. Wang, M. Waje, X. Wang, J.M. Tang, R.C. Haddon, Y. Yan, Nano Lett. 4 (2004) 345. [2] Y. Shao, G. Yin, Y. Gao, P. Shi, J. Electrochem. Soc. 153 (2006) A1093. [3] A.B. Fuertes, S. Alvarez, Carbon 42 (2004) 3049. [4] T.W. Kim, I.S. Park, R. Ryoo, Angew. Chem. Int. Edit. 42 (2003) 4375. [5] Z. Li, M. Jaroniec, Y.J. Lee, L.R. Radovic, Chem. Commun. (2002) 1346. [6] S. Yoon, G. Chai, S. Kang, J. Yu, K. Gierszal, M. Jaroniec, J. Am. Chem. Soc. 127 (2005) 4188. [7] T. Hyeon, S. Han, Y. Sung, K. Park, Y. Kim, Angew. Chem. Int. Edit. 42 (2003) 4352. [8] J.B. Joo, P. Kim, W. Kim, J. Kim, N.D. Kim, J. Yi, Curr. Appl. Phys. 8 (2008) 814. [9] F.P. Hu, Z. Wang, Y. Li, C. Li, X. Zhang, P.K. Shen, J. Power Sources 117 (2008) 61. [10] Y. Mi, W. Hu, Y. Dan, Y. Liu, Mater. Lett. 62 (2008) 1194. [11] W. Stöber, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [12] Z. Zhou, S. Wang, W. Zhou, L. Jiang, G. Wang, G. Sun, B. Zhou, Q. Xin, Phys. Chem. Chem. Phys. 5 (2003) 5485. [13] C.A. Bessel, K. Laubernd, N.M. Rodriguez, R.T.K. Baker, J. Phys. Chem. B 105 (2001) 1115. [14] W. Zhou, S. Song, W. Li, Z. Zhou, G. Sun, Q. Xin, K. Poulianitis, S. Kontou, P. Tsiakaras, J. Power Sources 131 (2004) 217. [15] X. Sun, Y. Li, Angew. Chem. Int. Edit. 43 (2004) 597. [16] M. Inagaki, Y. Okada, V. Vignal, H. Konno, K. Oshida, Carbon 39 (1998) 1706. [17] P. Kim, J.B. Joo, W. Kim, J. Kim, I.K. Song, J. Yi, Catal. Lett. 112 (2006) 213. [18] F. Mnaldonao, C. Moreno, J. Rivera, Y. Hanzawa, Y. Yamada, Langmuir 16 (2000) 4367. [19] A. Sadezky, H. Muckerhuber, H. Grothe, R. Niessner, U. Pöschl, Carbon 43 (2005) 1731. [20] J. Jang, H. Yoon, Adv. Mater. 5 (2003) 2088.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Simple synthesis of graphitic porous carbon by hydrothermal method for use as a catalyst support in methanol electro-oxidation Ji Bong Joo a, You Jung Kim a, Wooyoung Kim a, Pil Kim b, Jongheop Yi a,* a
School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul National University, Shinlim-dong, Kwanak-gu, Seoul 151-742, South Korea School of Environmental and Chemical Engineering, Specialized Graduate School of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Deokjin-dong, Deokjin-gu, Jeonju 561-756, South Korea b
a r t i c l e
i n f o
Article history: Received 8 March 2008 Received in revised form 15 August 2008 Accepted 20 August 2008 Available online 12 September 2008 Keywords: Carbon Electrocatalyst Hydrothermal method Graphitic Methanol
a b s t r a c t Graphitic porous carbon (GP-carbon) was prepared using non-toxic and economical carbon precursor (sucrose) by a simple hydrothermal method under a relatively low pyrolysis temperature. Uniform silica particles (100 nm) were used as pore-forming templates and etched by HF solution after carbonization. The GP-carbon exhibited highly graphitic properties with surface area of 425 m2/g and pore volume 0.42 cm3/g. The GP-carbon showed superior performance as a catalyst support in methanol electro-oxidation. The enhanced activity would be closely related to advantageous properties resulting from highlydeveloped graphitic structure of the GP-carbon. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Graphitic carbons have attracted much attention by many scientists and engineers in practical and fundamental researches for use in hydrogen storage, catalysis and electrochemistry, due to their high electronic conductivity and mechanical strength. Practically, graphitic carbon materials, such as graphite powder, carbon nanotube (CNT) and graphitic carbon nanofiber (GNF), have high conductivities and unique electronic properties which affect electron transfer and metal orientation. They showed the enhanced electrochemical performance in many reactions [1,2]. Although there are superior electronic properties and electrochemical performance, the use of the graphitic carbons have been limited due to the disadvantageous characteristics, such as low surface area and uncontrollable morphologies [3]. In addition, synthetic methods are complex and preparation conditions are uneconomical and severe. Thus, many researchers have made effort to fabricate graphitic carbon with various morphologies using economical methods. Generally, graphitic carbons have been prepared by pyrolysis using graphitizable carbon precursors such as polymer [3], aromatic hydrocarbon [4] or mesophase pitches [5,6] at an extremely severe condition. Yu et al. reported that ordered macroporous carbon with a graphitic framework was synthesized from a mesophase pitch as carbon precursor through pyrolysis at 2500 °C [6]. Although the ordered macroporous graphitic carbon was prepared, the pyrolysis * Corresponding author. Tel.: +82 2 880 7438; fax: +82 2 885 6670. E-mail address: [email protected] (J. Yi). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.08.031
temperature was still high. Several research groups have also reported preparation methods for graphitic carbons under mild conditions. Hyeon and co-workers prepared graphitic carbon nanocoils by carbonizing resorcinol–formaldehyde gel doped with metal salt, at a relatively low temperature. The prepared graphitic nanocoil showed higher performance than commercial carbon black for use as catalyst support in methanol electro-oxidation [7]. Hydrothermal synthesis is a simple and easy process for the preparation of powder materials using aqueous solution as a starting material. This method is also suitable for the control of crystal growth and mass production at a low temperature. Recently, nanostructured carbon materials, such as carbon sphere and porous carbon, have been synthesized by the hydrothermal synthesis route [8–10]. However, few investigations have been reported for the preparation of graphitic carbon materials which have a variety of morphologies. In this work, an economical and easy method for the preparation of GP-carbon under mild synthesis conditions is reported. We prepared highly graphitic GP-carbon using a simple hydrothermal method with templating route. The resulting material was characterized by SEM, HR-TEM, XRD and Raman spectroscopy, and examined for use as a catalyst support in methanol electro-oxidation.
2. Experimental The GP-carbon was prepared by a simple hydrothermal method using uniform-sized silica particles as templates under mild
268
J.B. Joo et al. / Catalysis Communications 10 (2008) 267–271
pyrolysis conditions. Uniform-sized silica particles were synthesized as described by Stöber et al. [11]. Fig. 1 illustrates the preparation scheme for GP-carbon synthesized in this study. Silica particles (100 nm diameter) were well dispersed in de-ionized water by ultrasonication for 3 h. Sucrose (Aldrich) and iron salt (Fe(NO3)3 9H2O, Sigma) were dissolved into the above aqueous solution under vigorous stirring. The aqueous solution was charged in a teflon-coated stainless steel autoclave. The autoclave was placed in a convection oven at 190 °C for 10 h with vigorous stirring. After cooling down the autoclave at room temperature, the black precipitates were filtered and dried to obtain polymer–silica composites. These polymer–silica composites were carbonized at 900 °C for 5 h and then carbon–silica composites, which contained the graphitic structure, were formed. Final GP-carbon was obtained by dissolution of the silica template and removal of Fe species with diluted HF and HNO3 solutions, respectively. For comparison purposes, sucrose was polymerized under ambient conditions without hydrothermal treatment to obtain amorphous porous carbon (APcarbon). The supported Pt catalyst (Pt/GP-carbon) was prepared by formaldehyde reduction method reported by Zhou et al. [12]. The Pt precursor (H2PtCl6, Acrose) was dissolved in 500 ml of de-ionized water and the prepared carbon support was well dispersed by ultrasonication. The concentrated formaldehyde solution as a reducing agent was added dropwise to the above solution and the batch was heated up to 70 °C under vigorous stirring for 5 h. The black precipitates were filtered, washed with copious amount of de-ionzed water and dried at 80 °C in vacuum conditions. The Pt loading was adjusted to 60 wt.% in the final catalyst. X-ray diffraction (XRD) patterns were recorded using a M18XHF-SRA diffractometer (MAC science). The macroscopic morphology and lattices of carbon support were investigated by field emission scanning electron microscopy (FE-SEM, JEOL, JSM6700F) and high resolution transmission electron microscopy (HR-TEM, JEOL, JEM-3010). Raman spectroscopy (Raman, RM 1000 Renishaw) was carried out to investigate the degree of crystalline structure of carbon support. The electrocatalytic activity in methanol electro-oxidation was investigated in a conventional half cell by cyclic voltammetry using EG&G 263A potentiostat. Pt gause and saturated calomel electrode (SCE) were used as a counter and a reference electrode, respectively. The working electrode was prepared by coating the catalyst ink on a disk-type glassy carbon electrode. Cyclic voltammograms
were recorded in 0.5 M H2SO4 solution containing 1 M CH3OH at room temperature. 3. Results and discussion Fig. 2 shows SEM and HR-TEM images of the prepared GP-carbon. As shown in Fig. 2a, partially broken pores were randomly distributed in the carbon framework, and the carbon had an openpore structure. The spherical pores with an average pore diameter of 100 nm were easily observed in the high magnitude SEM image (Fig. 2b). As described in Fig. 1, pores were formed by silica-templating route. During the hydrothermal process, the sucrose was polymerized on the surface of randomly distributed silica particles in the synthetic solution and silica templates were located in the polymer framework after polymerization of sucrose. After carbonization and silica etching, silica particles were removed by HF solution and regular spherical pores were generated. It should be noted that the pore of support material has an important role in many catalytic reactions. In electrocatalytic reaction, large pore of carbon support has many advantages. It is well known that electrocatalysis in a electrochemical system, such as fuel cells and battery, is taken place in the interfaces of the reactant, the catalyst surface and the electrolyte, i.e., a triple-phase boundary [13]. The triple-phase boundary would be easily formed in the relatively large pore, compared to micropore of support materials because the conventional polymer electrolyte has large molecular size. Thus, the carbon support with regularly large pore is more desirable in view of the mass transfer and would show considerably high performance in practical fuel cells. The (0 0 2) lattice fringes of the graphitic structure are clearly observed in HR-TEM images indicating that GP-carbon has highly graphitic characteristics (Fig. 2c). The d002 was measured and 0 found to be ca. 3.4 Å A. In addition, the GP-carbon exhibited a relatively high surface area (425 m2/g) and a large pore volume (0.42 cm3/g). X-ray diffraction patterns of the prepared carbons are shown in Fig. 3. In the XRD patterns of Vulcan XC-72 (Cabot) and AP-carbon, a broad peak at ca. 2h = 25° was observed, while the GP-carbon showed a sharp peak at the same angle which was caused by the (0 0 2) diffraction plane of the hexagonal structure of graphitic carbon [9,10]. The graphitic crystalline size was measured using Scherer formula and0 d002 spacing for G-carbon was calculated to be 9.8 nm and 3.35 Å A, respectively. The d002 spacing value of G-car-
Sucrose
Silica Fe species
Hydrothermal treatment and carbonization
GP- carbon
Silica etching
Fig. 1. Scheme of preparation procedure for GP-carbon.
Graphitization
J.B. Joo et al. / Catalysis Communications 10 (2008) 267–271
269
Fig. 2. (a), (b) SEM image and (c) HR-TEM image of the prepared GP-carbon.
Fig. 3. X-ray diffraction patterns of GP-carbon, AP-carbon and Vulcan XC-72.
bon is similar to that investigated from HR-TEM. These results imply that GP-carbon is a graphitic carbon with high crystallinity, while Vulcan XC-72 and AP-carbon are amorphous carbons with few graphitic properties [14]. Generally, graphitic structure was generated by pyrolysis of aromatic polymer under high temperature. In this work, porous carbon with graphitic framework was prepared under mild synthetic conditions. From experimental results, it could be noted that the polymerization of sucrose and subsequent aromatization were occurred through the catalytic action of Fe species during hydrothermal treatment. The graphitic structure of GP-carbon was effectively formed by a subsequent pyrolysis step. Hydrothermal treatment has also been reported to induce aromatization of polymers [15], and polymers have been easily converted to graphitic carbon in the presence of metal or metal oxide [16]. Graphitic characteristics of GP-carbon were further confirmed by Raman spectroscopy (Fig. 4). For comparison purposes, the Raman analysis of multi-wall carbon nanotubes (MWCNTs), which have highly graphitic properties, was carried out under same conditions. All carbon materials exhibit two distinct bands at around 1350 cm1 (D-band) and 1580 cm1 (G-band), while a peak at ca.
2720 cm1 (G0 -band or 2D-band) was observed in GP-carbon and MWCNTs. The D-band is closely related to the disordered scattering, which results from the imperfection or loss of hexagonal symmetry of the graphite structure. However, the G-band is observed in both amorphous carbon and graphite [17,18]. It should be noted that the G0 -band is only observed in highly-developed graphitic carbon [19]. Generally, the ratio of the D-band area to the G-band area (AD/AG) is expressed as an index of the degree of graphitization [18,20]. The AD/AG values of GP-carbon, MWCNTs, Vulcan XC-72 and AP-carbon were 1.1, 1.03, 1.65 and 2.1, respectively. The AD/AG value of GP-carbon was very close to that of MWCNTs. Importantly, the G0 -band peak was clearly observed in GP-carbon, and the intensity of the G0 -band was high as seen in Fig. 4. This indicates that GP-carbon had highly graphitic characteristics and retained its graphitic properties, which are comparable to those of MWCNTs. The feasibility of the prepared GP-carbon for the use as a catalyst support in methanol electro-oxidation was investigated. Fig. 5 shows a representative cyclic voltammogram of the Pt/GP-Carbon (inset figure) and voltammograms of the prepared Pt catalysts during the anodic scan. As shown in inset image, the current for
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Fig. 4. Raman spectra of GP-carbon, AP-carbon, MWCNT and Vulcan XC-72.
Fig. 5. Cyclic voltammograms of the prepared Pt catalysts (60 wt% loading) in 0.5 M H2SO4 containing 1 M CH3OH (scan rate = 30 mV/s).
methanol electro-oxidation slowly increased between 0 V and 0.3 V. The current increased dramatically after 0.3 V and maximum peak current was observed at ca. 0.7 V. The current then decreased because the number of active sites for methanol electro-oxidation had declined due to the formation of Pt oxide. After 0.8 V, the current re-increased, indicating that methanol electro-oxidation was occurring on the Pt-oxide surface. In the cathodic scan, the active sites were recovered due to the reduction of Pt oxide, which produced a methanol electro-oxidation current peak. The catalytic activities of electrocatalysts were generally evaluated by comparing the onset potential and current density on the anodic sweep. In this work, catalytic activities in methanol electro-oxidation were
compared using the current density during the anodic sweep. The Pt/AP-carbon had a slightly lower current density than the Pt-commercial catalyst (ETEK). However, the Pt/GP-carbon had the highest anodic current density among Pt catalysts. These results indicate that Pt/GP-carbon have the highest catalytic activity in methanol electro-oxidation. Since same amount of carbon-supported Pt catalyst, which has identical amount of Pt on the catalyst, was used on the working electrode, the variation in catalytic activity can be explained by the attribution to the effect of support material. Carbon materials with a graphitic structure have been frequently used in low temperature fuel cells and showed the
J.B. Joo et al. / Catalysis Communications 10 (2008) 267–271
unexpected enhancement in catalytic performance. The exact reason of the enhancement has been still controversial. However, it is believed that the unique electronic properties and the crystallographic orientation of graphite structure had positive effects on electrocatalysis. In this work, graphitic porous carbon was easily prepared using sucrose under mild synthetic conditions and showed the superior performance as a catalyst support in methanol electro-oxidation. From experimental results, it can be concluded that the highly graphitic nature of GP-carbon had positive effects on catalytic function of supported Pt catalyst, resulting in the high catalytic activity of Pt/GP-carbon in methanol electrooxidation.
4. Conclusions GP-carbon was prepared by a simple hydrothermal method at a relatively low carbonization temperature. The prepared GP-carbon was used as a catalyst support for a Pt catalyst in methanol electrooxidation. The GP-carbon exhibited highly graphitic characteristics and Pt/GP-carbon showed the superior performance in methanol electro-oxidation. The high activity is closely related to the unique properties of graphitic carbon, which affect on positive effect on catalytic function. The preparation method suggested in this work is a simple and efficient process for the preparation of graphitic carbon in mild conditions using low-cost materials, such as sucrose, which is non-toxic and easy to handle. Therefore, the method described in this work would be an easy and environmentalfriendly technique for the production of graphitic porous carbon.
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